Locating bottom of oil production



D. SILVERMAN LOCATING BOTTOM 0F OIL PRODUCTION Sept. 1l, 1945.

Filed Jan. 14, 1942 Patented Sept. 11, 1945 UNITED STATES PATENT OFFICE I 2,384,648 e i.ooATiNG BOTTOM or on. PRODUCTION Daniel Silverman, Tulsa, Okla., assigner to 'Stanolind Oil and Gas Company, Tulsa, Okla., a cor.- poration oi' Delawal'tk,` Application January 14, 1942, Serial No. 426,714 e claims. con. 1ra-182) This invention pertains to the art of determining the location of regions of entry of oil or similar non-conducting uids into a well producing procedure;

salt water or the like, so that the production obl tained is a mixture of the two liquids. Obviously it is highly desirable to eliminate, as far as can be, the production of the undesired liquid. After the points of water entry have been determined by one means or another various cementing methods are used to stop entry of the water into the well. If the points of entry of the oil and the salt water are widely separated the problem vis a simple one. Generally the points of entry of each uid are only approximately known and quite often they 4 are immediately adjacent so that often the flow of oil is affected by operations tending to cut down the flow of water.

It is an object of this invention to provide a method for determining the location of the lowest point of entry of the oil so that by comparison with the data on the'location of the points of entry of water a decision can be made as to the feasibility of trying to reduce the water Without ailecting the oil. It is a further object of this invention to provide means whereby the lowest point of Oil entry into a well can be determined accurately, rapidly, and without the use of complicated equipment in the well.

In some instances it is desirable to know the location of the point of entry of the oil in a well in which no salt water is present. It is an object of this invention to provide a method and means for determining the point of oil entry in such instances. Further objects and advantages of this invention will be apparent from a perusal of the accompanying specification.

In orderA to illustrate the invention several drawings accompany this specification and form a part thereof. They illustrate one simple embodiment of the invention. There is no intent to limit the scope of the invention to the apparatus illustrated and described. In these figures the same reference numeral in several gures refers to the same or a corresponding part.

In these drawings, Figure 1 represents in diagrammatic form a cross section of a well penetratlng earth formations including at least one oil Figure 3 is a representation of a chart obtained by use of the apparatus shown in Figure 1.

In brief this method might be described as stopping the i'low of well fluids into the well, displacing all of the fluids in the well to a point above the suspected region of entry of the oil by a relatively non-conducting liquid, the density of which exceeds that of the salt water in the Well and which is miscible with the oil in the well, thereafter causing well fluids to ilow into the well, stopping the now of the fluids into the well to produce a period of quiescence during which the water which has been produced (or added if the well does not produce salt water) settles to form an interface defining the lowest point of entry of the Oil, and measuring during the period of quiescence the conductivity of the liquids in the well as a function of depth in order to determine this interface.

This is illustrated in part by Figure 1. In this ligure a well indicated generally at II has been drilled from the surface of the earth I2, penetrating formations I3 to I6. Of these formations I4 is assumed to be an'oil producing formation and I6 is assumed to be a water producing formation. As will be apparent from further discussion it is not necessary that these formations occupy the relative positions shown. Casing Il has been cemented by cement I8 into the well down to a certain point in the relatively impervious formation I3. Below this point the walls of the well are uncased and form the producing region of the well. Tubing I9 has been set in the well and held in place by a casing head 20 which is firmly attached to the top of the casing I'I. Valved lines 2l and 22 form a means of access to the annular space between the casing II and the tubing I9. The upper end of the tubing itself terminates in a cross 23, one arm of which is closed by a valved application to use in this kind of a well. but that it can be used to equal advantage in the testing of a well which has passed its flowing life and which is either pumped or prodouced by gas lift or the like.

As a ilrst step in the preparation of the well for th'e test the plug normally closing the top oi' cross 23 is replaced by the stufllng box 20. 'I'he valves in lines 2|, 22, and 24 are then closed, stopping the flow of well fluids into the well. A cable 21 is passed through the stumng box 2i and lowered into the well until the two electrodes 2l and 22 are in the vicinity of the uncased portion of the well. This cable 21 contains at least two conductors which are insulated from each' other. Thus, for example, it may consist oi a single eentral conductor surrounded by a sheath of insulation which in turn is surrounded by an outer metalllc sheath forming the second conductor, or two insulated conductors surrounded by a nonmetallic sheath may be used if desired. Also a greater number of conductors can be used, although this is normally not desirable. Each' of the two conductors is separately connected to one of the two electrodes 28 or 29 so that these electrodes can be connected through the cable to apparatus at the top of the well.

At the top of the well the conductors are connected to a battery 30 or other source of potential and a resistor 3l completing th'e circuit. Normally the cable 21 is carried upon a reel in a manner well known in the art with the ends of the conductors brought out to slip rings on the reel against which brushes connected to the battery 30 and the resistor 3| are connected. The circuit including th'e two electrodes 28 and 2l is arranged in such a fashion that the flow of electricity through the resistor Il depends upon the conductivity of the fluid between the two electrodes. The drop of voltage across this resistor 3l therefore increases as the conductivity between th'e electrodes increases and vice versa. Normally the electrodes 28 and 28 are spaced relatively closely together, of the order of 3 to 12 inches apart. However, closer spacing of the order of an inch or less, or wider spacing up to the order of several feet may be used if desired.

Preferably after the electrodes have been placed in the encased portion of the well, but not necessarily so, a conditioning liquid is pumped into the well. This conditioning liquid should be at least a fair electrical insulator and miscible with the well fluid which is to be located, but which is immiscible with water. Organic liquids such as carbon tetrachloride, carbon disulilde, chloroform, bromoform, the trichlor and tetrachlor ethylenes, and the corresponding bromine compounds, etc., are usable in this connection. The liquid must be one which' is heavier than salt water. Mixtures of such materials with other lighter liquids can be employed. Thus, for example, I prefer to use a mixture of carbon disulfide or carbon tetrachloride with a petroleum oil in such proportions that the density of the resultant liquid is slightly heavier than that of the salt water in the well, that is. so that the specic gravity of the liquid exceeds that of salt watei but by not more than about 30% maximum. This liquid is pumped into the well through any o f the valved lines 2|, 22 or 24. Because it is of greater density than th'at of the fluids in the well, it sinks to the bottom and displaces the well fluids above it. If desired the liq-l Vuid can be introduced into the well by means of a dump bailer or any other means of introducing a liquid into a well known in the art. An interface l2 between this liquid and the well fluids is formed. Enough of the liquid should be used to insure that all of the well fluids have been displaced above the region in which it is expected that oil is present. This can be secured by pumping in a suiilcient volume of the conditioning liquid to iill the uncased portion of the well. If the specific gravity is kept to a value which is only slightly greater than that of the salt water in the well, it is apparent that the hydrostatic head of the liquids in the well will be almost the same as that which existed before the conditioning liquid was pumped in so that there will be little tendency for this conditioning liquid to leak into the permeable formations and thus be lost. After the well iluids have been displaced to a point above the region of ingress by this non-conducting high density liquid, the well is caused to flow (at a relatively low rate preferably) by opening one of the valved lines 2 I, 22 and 24. This causes the various well fluids to ilow through the conditioning liquid present up past the interface 22 between the well fluids and the conditioning liquid, due to the low specific gravity of these well iluids relative to that of the conditioning liquid. Since the salt water is relatively immiscible with the conditioning liquid, there will be little effect on this liquid due to the passage of the salt water. On the other hand the oil which is nowing through the liquid is miscible with it, and therefore dilutes the conditioning liquid. 'I'his dilution continues as long as the ilow exists and extends upward from the lowest point of oil ingress into the well. Since the speciilc gravity o! oil is less than that of water the specinc gravity of the diluted conditioning iluid decreases until it is less than that of the salt water. Th'e operator causes the well iiulds to ilow into the well until he is sure that the well conditioning liquid above the point of lowest oil entry has been diluted to such a point that the specific gravity is less than th'at of the salt water. Production vfrom the well is then stopped by closing the valve which has been opened, causing the flow of the iluid from the permeable formations into the uncased portion of the well to cease. 'I'his :tiow is stopped for an appreciable period of quiescence. During this time the salt water in the well which is of greater specific gravity than the diluted conditioning liquid will displace this diluted conditioning liquid by gravitation and thus form an interface ll (Figure 2) with the non-diluted conditioning liquid at the lowest point at which the dilution has occurred. Such period depends upon the relative densities of th'e diluted conditioning fluid, and the salt water, and must be, of course, sufficiently long to permit the salt water substantially completely to replace the diluted conditioning liquid under the influence of the force of gravity. Accordingly, the time required for this appreciable period of quiescence cannot be specifically given in terms of minutes or hours but preferably the time should exceed the order of 20 minutes. The longer this appreciable period of quiescence, the more certain is th'e gravitational separation. On the other hand. excessive periods of quiescence, such as the order of 8 hours or more, will normally give erroneous results in that there is suillcient time to permit some of the undiluted conditioning fluid to settle into the formations adjacent the well. This obviously is the lowest point of oil entry into the well. I t is apparent that it makes no difference whether the salt water in the well was produced responding to this lowest point oi' oil entry.

The operator now prepares to locate the position of this interface.

In order to accomplish' this result a recording voltmeter or the like is connected across the resistor 3i. In Figure l, one such type of recording voltmeter is shown. In this figure the drop across resistance @I is applied between th'e grid and cathode of a vacuum tube 3l. 'I'he filament of this tube is energized by a storage battery or other suitable supply of potential which is connected across the points r-. A suitable biasing resistor 35 is employed. The plate of this vacuum tube 35 is connected through a, plate resistance 3d to a battery 3l or other source of direct potential which is in turn connected to the cathode of vacuum tube 3d. In order that the steady platecurrent of the vacuum tube Si need not affect the reading of the recording voltmeter, a second vacuum tube circuit is employed. This includes a vacuum tube 38 preferably of the same type as that of vacuum tube te, the cathode of which is connected to the cathode of the vacuum tube Sli through a variable .resistance Se. The grid of this tube 3B can also be connected to the cathode of the vacuum tube 8d. The plate of tube it is connected through a resistance do, which preferably is of the same size as resistance 3d, to the positive side of battery 3i. A recording milliammeter ti or the like is connected across th'e plates of the two tubes 318 and se.

Prior to operation, with no drop of potential across the resistance 3i, resistor d is adjusted so that the drop of potential across resistance d is equal to the drop of potential across resistance te so that the voltmeter di reads zero. The equipment is now set to operate.

The electrodes 23 and 28 are 'lowered through the uncased portion of the well and the recording milliammeter records on a moving chart 62, the amplified drop of potential across the resistance si. As soon as the electrodes 23 and 29 are immersed in the salt water there will be a large drop of potential across the resistance 3i and a corresponding deflection of the recording voltmeter di. However, as soon as the electrodes pass the interface 33 they pass into the relatively high resistance conditioning liquid which has been placed in the well; and the flow of current through resistance 3i abruptly decreases, thus decreasing the corresponding deflection of the voltmeter di. The resultant chart will appear approximately as shown in Figure 3 with a large change in deflection at the point A which corresponds to the point at which the electrodes passed the interface 33 between the conducting brine in the well and the relatively non-conducting conditioning liquid. A record is made of the mov..ment of the chart di as the depth oi' the electrodes 28 and 28 is varied in the well, in a manner well known in the well logging art. Therefore it is possible to correlate the point at which the deflection changed abruptly with the elevation of th'e electrodes in the well at that instant. By this means the position of the interrace 33 can be accurately determined. 'I'he location of the interface concludes the test. 'I'he well can then be put back on production, or steps taken to isolate the water, etc.

point at which the conductivity of the liquids in the well changes as the interface 33 is passed.

.Any type of conductivity determining apparatus known in the prior art can be used for this pur- POSe.

It is occasionally found that if particular care is not used in employing a conditioning liquid the specific gravity of which is very close to that of the brine in tbe fwell, that a certain amount of the liquid will ieak into the fonmations, causing diiculty in determining the location of the interface 3 3. In such cases it is advantageous to reduce the permeability of the formations at the point at which they form the walls of the well by circulating into the well ,drilling fluid or an aqueous suspension of bentonite or the like which is .pumped into the well through one of the valved lines in the tubing or casing and pumped out through the other line, for example, in through line 22 and out through line 24. By this means a illter cake is built up on the walls of the well thus decreasing the permeability of the 'formations and tending to stop the flow of the conditioning liquid into the well formations. 'I'his reduction of permeabilityis carried out before the well is conditioned with the high resistivity conditioning liquid, as previously described. It should be emphasized, however, that the filter cake which is built up does not, of course, decrease the permeability of the formations to zero, that is, it does not seal oi the well formations from the :well. On the other hand this does not affect the result of the test if suflicient time is allowed to elapse so that the well fluids can seep into the well while the early steps of this procedure are being carried out.

If the well has passed through the flowing period and is either on the pump or being produced by gas lift, the same type of procedure is carried on rwith the exception that in this case instead of opening a valve at the surface in order to cause the formation fluids to flow into the well it is necessary to pump some of the well fluids out of the lwell or to gas lift these materials. This is the only change in the test technique which need be employed in order to carry out the invention in that type of well. If there is no salt water produced naturally in the well, salt water can be pumped into the Well after the conditioning liquid has been placed. The procedure is otherwise as previously described.

Various changes and modifications of the apparatus used in determining the location of .the interface can be employed. Other variations in the entire testing technique will be apparent to those skilled in the art. The invention is not limited to the particular embodiment described but is best set out in the appended claims.

I claim:

l. A method of determining the lowest region of entry of a relatively non-conducting liquid less dense than water into a well, comprising stopping the flow of well fluids including salt water into said well, displacing said Well fluids in said well to a point above said region of entry .by a. conditioning liquid which is immisible with -water and miscible with said relatively non-conducting liquid, and an electrical characteristic of which is markedly diierent from that of salt water, the density of said conditioning liquid exceeding that of the salt water in the well, andthe density of said conditioning liquid when sufficiently diluted with said relatively non-conducting liquid being less than that of said salt water. causing the flow of well fluids into said well for such time that a portion of said conditioning' liquid will be diluted with said relatively non-conducting liquid and will have a density less than that of said salt water, again stopping the i'iow of said well fluids into said 'well to establish a substantial period of quiescence sufficient to permit replacement of diluted conditioning liquid by salt water from said well fluids, whereby an interface is formed between said salt Iwater and the undiluted portion of said conditioning liquid. and measuring after said period of quiescence the electrical characteristic of the liquid materials in said well as a function of depth.

2. .A method of determining the lowest region of entry of oil into an oil fwell, comprising stopping the -iiow of well fluids including salt water into said well, displacing said well fluids in said well to a point ,above said region of entry by a relatively non-conducting liquid which is immiscible with water and miscible with oil, the density of said liquid exceeding that of the salt water in the swell, and the density of said liquid when suiilciently diluted with oil being less than that of said salt Water, causing the flaw oi' Well fluids into said well, whereby oil and salt rwater flow into said well and said oil will dilute a portion of said non-conducting liquid until the density is less than that of said salt water, again stopping the flow of said well fluids into said well to establish a substantial period of quiescence sumcient to permit replacement of diluted non-conducting liquid yby salt water from said well fluids |whereby an interface is formed between said salt water and the undiluted portion of said non-conducting liquid and measuring after said period of quiescence the conductivity of the liquid materials in said well as a function of depth.

3. A method of determining the lowest region of entry of oil into an oil well, which also produces salt water comprising stopping the flow of well fluids into said well, displacing said well 'fluids in said well to a -point above said region of entry by a high resistivity liquid which is immiscible with water and miscible with oil, the density of said high resistivity liquid exceeding that of the salt water in the well, and the density of said liquid when sumciently diluted -with oil being less than that of said salt rwater, causing the flow of oil and salt 'water into said well from the surrounding formations for such time that a portion of said high resistivity liquid will 'be diluted with oil and will have a density less than that of said salt water, again stopping the flow of said well fluids into said well to establish a substantial period of quiescence suflicient to permit gravitational replacement of diluted high resistivity liquid by salt water from said 'well fluids, whereby an interface is formed between said salt water and the undiluted portion of the high resistivity liquid and measuring at the end of said period of quiescence the conductivity of the liquids in said well as a function of depth, whereby the position of said interface is determined.

4. A method of determining the lowest region of entry of oil into an oil well, which also pro duces salt water comprising stopping thefiow of swell fluids into said well, displacing said well fluids in said well to -a point above said region of entry by a high resistivity, water immiseible heavy solvent for oil, the density of which is greater than that of the salt water in the well and the density of said solvent when sufficiently diluted with oil being less than that of said salt water. causing the flow of oil and salt water into said well from the surrounding formations for such time that the solvent will be diluted with oil and will have a density less than that of said salt water, again stopping the flow of said well fluids into said well to establish a period of quiescence sufficient to permit gravitational replacement of diluted solvent by salt water from said well fiuids, whereby an interface is formed between said salt water and the undiluted high resistivity solvent and measuring and recording after completion of said gravitational replacement a function of the resistance between two electrodes lowered through the liquids in said fwell, from which the position of said interface is determined.

5. A method of determining the lofwest region of entry of oil into a well also producing salt water which comprises decreasing the permeability of the walls of said well in the uncased portions thereof, stopping the flow of well fluids into said well, displacing said well fluids in said weli to a point above said region of entry by a high resistivity liquid, the specific gravity of which is greater than that ofthe salt water in the well, but less than 1\.3, said high resistivityliquid being immisclble Iwith water and miscible with oil. and the specific gravity of said liquid when sufiiciently diluted with oil being less than that of said salt water, causing the flow. of oil and salt water into said well from the surrounding formations .for a period of time suicient to permit a portion of said liquid to be diluted with oil and have a specific gravity less than that of said salt water, again stopping the flow of said well fluids into said well to establish a substantial period of quiescence sumcient to permit gravitational replacement of diluted liquid by said salt -water from said well fluids, whereby an interface is formed between said salt water and the undiluted liquid,

and measuring after said period of quiescence and after the substantially complete gravitational 

